do a voltage drop test on all wiring. If you did aircraft electrical stuff, you know how to do that. This assumes of course that you've not already done it.
20v is low. Remember a VR is also a rectifier, and a rectifier cuts the ac voltage basically in half, so I like to see 30v+ out of dynamo's (no load full throttle). On my mom's g1800, just got through playing with the electrical system on it and the dynamo puts out around 60VAC at maximum rpm (3600 engine rpm, and remembering that the crank pulley is larger than the dynamo pulley meaning the dynamo turns more rpm than the crankshaft). Also remember, the dynamo output is AC voltage (I think you know that already) and if you test it on DC meter setting it will show "off".
if it were mine (and it's not) I'd put a small Kubota alternator on it and make the wiring changes & be done with it. Those little alternators are like $40 or something on scamazon. You'll also need the spacer, get it from a bx1800 or similar, same for the little alternator. Nothign wrong with the dynamo, they're just weak as far as amperage output and the regulators aren't set up to charge all that well either. I like to see 14v-14.5v and most of the kubota dynamo'regulators will only do 13.5-13.8. It works but it's not ideal. In the case of my mom's little mower, 14A is plenty. It doesn't have much of an electrical draw while running. The lift pump and hourmeter but that's about it.
oh, one last thought. I had a ZD326 show up once wasn't charging very well. Someone had the dynamo apart to put bearings in it (simple task) but didn't get the nut that holds the pulley to the shaft tight enough and it was slipping. That was interesting to find.
20v is low. Remember a VR is also a rectifier, and a rectifier cuts the ac voltage basically in half, so I like to see 30v+ out of dynamo's (no load full throttle). On my mom's g1800, just got through playing with the electrical system on it and the dynamo puts out around 60VAC at maximum rpm (3600 engine rpm, and remembering that the crank pulley is larger than the dynamo pulley meaning the dynamo turns more rpm than the crankshaft). Also remember, the dynamo output is AC voltage (I think you know that already) and if you test it on DC meter setting it will show "off".
if it were mine (and it's not) I'd put a small Kubota alternator on it and make the wiring changes & be done with it. Those little alternators are like $40 or something on scamazon. You'll also need the spacer, get it from a bx1800 or similar, same for the little alternator. Nothign wrong with the dynamo, they're just weak as far as amperage output and the regulators aren't set up to charge all that well either. I like to see 14v-14.5v and most of the kubota dynamo'regulators will only do 13.5-13.8. It works but it's not ideal. In the case of my mom's little mower, 14A is plenty. It doesn't have much of an electrical draw while running. The lift pump and hourmeter but that's about it.
oh, one last thought. I had a ZD326 show up once wasn't charging very well. Someone had the dynamo apart to put bearings in it (simple task) but didn't get the nut that holds the pulley to the shaft tight enough and it was slipping. That was interesting to find.